To understand the molecular basis of membrane excitation it would be highly desirable to apply spectroscopic techniques to single, living, voltage-clamped cells. A study of slow fluorescence responses of mammalian endplates is planned as an approach to this goal. This tissue offers very high concentrations of excitation-mediating membrane-embedded proteins. Single clamped endplates will be stained with one of three pharmacologically active fluorescent probes (quinacrine, snake -toxin or dansyl acyl choline), irradiated for brief periods with a minute spot of exciting light (about - 350 - 490 nm), and the emitted light collected with a high numerical aperature objective and measured. The variation in the light intensity with application of cholinergic agonists or antagonists is measured and used to deduce features of the drugs' actions. The probes have been selected because they have been successfully used in studies of cholinergic systems, and their actions are reasonably well understood.